Measurement of the corrosion rate of magnesium alloys using Tafel extrapolation

The hypothesis that the corrosion of Mg alloys can be adequately estimated using Tafel extrapolation of the polarisation curve is termed herein the electrochemical measurement hypothesis for Mg. In principle, such a hypothesis can be disproved by a single valid counter example. The critical review of Mg corrosion by Song and Atrens in 2003 indicated that, for Mg alloys, Tafel extrapolation had not estimated the corrosion rate reliably. This paper examines the recent literature to further examine the electrochemical measurement hypothesis for Mg. The literature shows that, for Mg alloys, corrosion rates evaluated by Tafel extrapolation from polarisation curves have not agreed with corrosion rates evaluated from weight loss and hydrogen evolution. Typical deviations have been ∼50-90%. These were much larger than the precision of the measurement methods and indicate a need for careful examination of the use of Tafel extrapolation for Mg. For research that nevertheless does intend to use Tafel extrapolation to elucidate corrosion of Mg associated with service, it is strongly recommended that these measurements be complemented by the use of at least two of the three other simple measurement methods: (i) weight loss rate, (ii) hydrogen evolution rate, and (iii) rate of Mg²⁺ leaving the metal surface. There is much better insight for little additional effort.